Phosphating ferrous substrates to produce an iron phosphate coating is an old and well-known art. Such coatings are typically employed to pretreat the surface of ferrous substrates prior to applying other protective coatings and aid in resisting corrosion.
Presently the two most often used methods of applying a phosphate coating to substrates are a spraying process and an immersion process. One drawback to the spraying process is that it is typically limited to a relatively light or intermediate iron phosphate coating. Heavier coatings applied by the spraying method result in a dusty or loosely adhered iron phosphate coat. A light coating is generally considered by those skilled in the art as about 35 mg per square foot or less. Intermediate coatings range from 35 mg to about 80 mg per square foot. The immersion process also provides a reasonably satisfactory iron phosphate coating for application of light and intermediate coatings. However, as one approaches the upper portion of this intermediate range, about 60 to 70 mg per square foot, the iron phosphate coatings tend to become less tightly held or bound to the substrate and "dusting" occurs. "Dusting" as referred to herein means an iron phosphate coat which is powdery and so poorly adherent or loosely bound to the substrate that the coating tends to become easily lost from the surface of the substrate. For those coatings referred to as heavy coatings by those in this field, that is above about 80 mg per square foot, the formation of powdery coating becomes sufficiently severe as to extremely limit use of such coatings for many applications. Further, in some instances such loosely held, powdery coatings represent a serious health hazard in the workplace as iron phosphate dust can become airborne and may effect personnel working in the exposed area.
Prior to the present invention, it was well known to those skilled in the art that chlorate and organic accelerated phosphating immersion baths produced heavy iron phosphate coating, however, such coating was loosely adhered particularly near the surface of the coatings. Inorganic accelerated baths tend to produce only relatively lighter coatings on a practical or cost efficient basis.
It has long been recognized that a heavier iron phosphate coating is highly desirable to improve corrosion resistance of the substrate with or without the application of an additional protective coating, such as a paint or oil coat. However, poorly adherent, powdery coatings are generally unacceptable as a paint pre-treatment step due to the poor adhesion of the paint to the coating. Further, loosely held particles of the phosphate coating which are easily dislodged from the surface during handling and the like, represent a significant waste of the deposited coating and reduce the effectiveness of the coating process. The deposition of a satisfactory, heavy, iron phosphate coating useful for a wide variety of applications has been a significant and long standing problem to those skilled in the art.
The immersion process is recognized as the preferred method to apply a heavier phosphate coat because of the better control of time and consistency of the contact between the substrate and the phosphating bath, as well as more cost effective control of the other operative bath parameters. However, prior to the present invention, a satisfactory immersion bath composition and method which provides a heavy, strongly adherent, non-powdery iron phosphate coat has eluded those skilled in the art.